Hello, welcome to my little Warwick guide.

A premise: Warwick is going to get a rework by the end of 2016, but from what I've heard about the changes that Riot has in mind, it may still be viable after the rework. Either way I'll update it as soon as possible.

I'm not a high ELO player, but I don't think that you need to be platinum to write a guide on Warwick. You just need to understand his possibilities, which are not that wide.
Warwick is a beginner level jungler, with little to no micro mechanical skill required and a very high sustain since level one due to his passive.

The playstyle that I suggest with Warwick is very aggressive on objectives, especially early dragons, and I recommend to gank or go counterjungling everytime your ultimate Infinite Duress is up.

After the changes to masteries I think Fervor of Battle is still better than Warlord's Bloodlust , although it's less effective than before.
Expose Weakness makes your ganks much more powerful and I'd chose Battle Trance over the others because you need some seconds to be effective anyways and you don't want to be oneshot mid-game because of Double Edged Sword .
In the Resolve tree I chose Explorer to gank more easily and Fearless to protect you for those initial 2 seconds before you start stealing life like crazy.

Warwick is a champ that doesn't have much outplay potential, he wins if he is stronger than the enemy or if the enemy fails to outplay him, but what he is good for, in my opinion, is taking objectives.
This build will help you early to take dragons and turrets and later to peel and lock down enemies in teamfights.

Core offensive items:

• Enchantment: Bloodrazor is doubtlessly the best enchantment for WW and the Skirmisher's Sabre is very useful to win 1v1 fights quickly. It gives you on-hit percent health damage.
• Blade of the Ruined King has always been a must have. It gives you AD, AS, lifesteal and more on-hit percent health damage it synergises perfeclty with Warwick's kit.
• Guinsoo's Rageblade is basically the new Enchantment: Devourer: after five attacks you start to proc on hit-effects twice every other basic attack, which means 30% max health damage every two autoattacks and a lot of healing with Bloodrazor and BOTRK.
  Furthermore both raw stats that Guinsoo's gives you are very useful to you, since your Hungering Strike scales off 100% of your AP.

Defensive items (situational):
I find that when your strength is your sustain, you'd better have as few HP as possible, but that they may not be taken away all at once: have a lot of armor and MR and few HP. That way you can regenerate a bigger percentage of your health with your lifesteal.

• Frozen Heart is a great option for Warwick, because it gives you CDR for your Q and R, a lot of mana and armor.
• Thornmail is very useful when against an AD autoattacker (such as Tryndamere, Master Yi or any ADC) who is doing well on the other team.
• Spirit Visage is probably your best option for MR, since it increases the healing from your lifesteal and passive by 25% and gives you that little HP that you may need not to get bursted down from the distance.
• Wit's End is a useful option since it has on-hit effects and may grant you up to 80 MR at full stacks, leaving the enemy completely vulnerable to your Q Hungering Strike and ultimate Infinite Duress, which deal magic damage.
• Zz'Rot Portal is very useful if you are roaming a lot and split pushing: it gives you MS in lane and lets you push a lane without even being there. Plus it gives you both armor and MR.
• Guardian Angel is useful especially if you decide to play as an assassin in teamfight and burst down the enemy carry. Not the best item though, in my opinion.

Situational items:

• Frozen Mallet is a very useful item if you realise that everybody escapes you even with your ultimate.
• Lord Dominik's Regards is useful against a tanky team composition, since you can just shred them at once.
• Mortal Reminder is a must have against a healer (such as Dr. Mundo, Fiddlesticks, Nasus) or if the enemy team has a healing support (like Soraka, Sona or Taric).
• Black Cleaver is useful against tanks, because it shreds their armor, but you should only buy it if you need the 20% CDR.
• Ravenous Hydra is recommended if you want to splitpush.
• Nashor's Tooth may be fun if you are ahead and want to deal more mixed damage to surprise them. It also deals on-hit magic damage, which will be amplified by Guinsoo's Rageblade.
• Hextech Gunblade is nice if you want some more raw attack stats and more healing. Its active may also be used to last hit distant enemies or to chase them.